I looked up both lambast and lambaste in several dictionaries, but came up with no conclusions about which one is AE and which BE (if this distinction can ever be made). Moreover, the different spelling entails different pronunciations: lambast [læmˈbæst] vs lambaste [læmˈbeɪst].
The Macmillan Dictionary s.v. states that this verb is mainly used in journalism, so I suppose that it’s a relatively recently-formed word.
Are there any diatopic variations between the two forms? That is, is one mainly British and the other mainly American?
If there are no such variations, which one is more correct or widespread in terms of pronunciation and spelling?
The Oxford English Dictionary lists lambaste from the 17th century, then notes lambust and lambast as alternate spellings dating only from the 18th century. My American dictionary also has lambaste.